Electrical power systems have relied on a system of protective relays to monitor and protect the expensive equipment used in electrical power transmission systems. Such systems have lines, transformers, busses, and feeders that supply power to the entire nation. Electrical faults caused by short circuits, lightning strikes or equipment failures cause disturbances on these systems. Protective equipment such as circuit breakers, line switches, capacitors and reactors operate automatically to cut off faults or to provide system stability as needed. To operate and monitor this equipment, a system of protective relays has been developed. These relays monitor current, voltage, frequency and other operating parameters and cause the protective devices to operate if a fault is detected. In the past, relays were electromechanical devices that had current and voltage detectors. Operation was mechanical: a rotating disk was energized by an overcurrent, for example. The disk then turned until a contact was made, causing the relay to trip, sending an alarm signal or a signal to operate a circuit breaker, for example. Relays were set to operate at the appropriate current level and in a specific amount of time. Today, many relays are electronic, they have programmable central processing units (CPU), volatile read write memory, typically referred to as "random access memory" (RAM), and input-output (I/O) circuits.
All relays must be set and tested before use. Testing of electronic relays follows the same basic course as was used for the mechanical designs. To test any relay, a test voltage or current is applied to the input circuits of the relay and its performance is monitored. For an overcurrent relay, for example, the current sent to the relay is gradually increased until the trip level is reached. Then, the time it takes for the relay to trip is monitored or set for the particular relay. For mechanical models, setting may take some time as it requires physical adjustments of the mechanism. In electronic relays, the settings can be programmed exactly and then tested using the test procedure described above.
To perform the tests, technicians or engineers use a test set that is connected to the relay circuits in the field. In this way, the circuitry of the system can be tested as well as the relay itself. An example of such a relay test set can be found in U.S. Pat. No. 4,177,419 to Fioentzis.
One of the features of the new digital electronic relays is that they are able to capture and record real-time event data. For example for any type of fault, the relay can record the voltages, currents phase angles, frequencies and other parameters as the event is unfolding. This data is useful in determining what caused the fault and whether the protection system operated properly during the event. In the past, this data was recorded on oscillographs, which produce a paper record of the event that could only be used for visual analysis. The storage systems available today store data in a portable computer readable form. Some relay test sets available today can read this stored data and then reproduce the stored waveforms in analog form. Although this is a marked improvement, using today's equipment to replay a stored event requires a test set that is capable of receiving the stored data and then converting it into the analog signals that the data represent in order to reproduce the archived event at the relay inputs. Since such test sets have high capital and operating costs, it is not always practical to test relays using stored event data.